add privacy-first analytics with progressive event collection

Three-layer pipeline: Plug for all HTTP requests (no JS needed), LiveView
hook for SPA navigations, JS hook for screen width. ETS-backed buffer
batches writes to SQLite every 10s. Daily-rotating salt for visitor hashing.
Includes admin dashboard with date ranges, visitor trends, top pages,
sources, devices, and e-commerce conversion funnel. Oban cron for 12-month
data retention.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

config/config.exs

@@ -93,7 +93,8 @@ config :berrypod, Oban,
     {Oban.Plugins.Cron,
      crontab: [
        {"*/30 * * * *", Berrypod.Orders.FulfilmentStatusWorker},
-       {"0 */6 * * *", Berrypod.Sync.ScheduledSyncWorker}
+       {"0 */6 * * *", Berrypod.Sync.ScheduledSyncWorker},
+       {"0 3 * * *", Berrypod.Analytics.RetentionWorker}
      ]}
   ],
   queues: [images: 2, sync: 1, checkout: 1]